Piezoelectric optical MEMS devices are used for sensing or manipulating optical signals on a very small size scale using integrated mechanical, optical, and piezoelectric and electrical systems. Examples of such devices include vibration monitors, precision positioners, optical switch, optical cross-connect, tunable VCSEL, and autofocus actuated lenses. These devices are typically fabricated using piezoelectric and conductive electrode materials along with optical and thin-film materials such as silicate glass, silicon, silicon dioxide and silicon nitride.
In piezoelectric optical MEMS devices, silicate glass, such as borophosphosilicate glass (BPSG) or phosphosilicate glass (PSG) is widely used as the lens material. Problems are sometimes encountered with silicate glass because of its high moisture absorbency. When moisture is absorbed by silicate glass used in a lens, the moisture can cause stress in the lens and cause optical properties of the lens, such as offset and transmittance, to drift over time. This, in turn, can result in yield and reliability problems. A conventional countermeasure to such problems is the coating of a moisture barrier on the top and bottom\back of the lens toward the end of the fabrication process.
A problem with this conventional countermeasure is that the subject lens is exposed to the fabrication environment and is also exposed to subsequent manufacture processing environments until the lens is sealed with the moisture barriers. The time period between the time that the lens is exposed to the fabrication and manufacturing environments and the time that the lens is sealed can be days or even weeks. During this period the lens is unprotected from moisture.
Lens fabrication includes formation of a disc shaped wafer, which contains multiple identical lens substrates arranged in adjacent relationship in a grid. This lens substrate grid of the wafer is, after performance of a number of processes on it, cut (“diced”/“singulated”) into individual lens components, typically with a wafer saw or stealth laser. One of the lens fabrication processes performed before singulation is forming multiple small, backside holes in the wafer to allow lens movement. (This formation of backside holes is generally referred to in the art as “backside cavity formation”.) After backside cavity formation the backside of the wafer must be coated with moisture barrier. This requires handling of the wafer. However, because of the multiple holes produced during backside cavity formation, the wafer has become fragile and more difficult to handle. The handling of a fragile wafer makes moisture barrier coating a difficult task.